It is known from French patent specification No. FR 2 455 221A to provide a motorised reduction gear unit which includes, between a driving shaft and a driven shaft (the driving shaft being driven by a motor and constituting for example the motor shaft itself), a geared transmission in the form of a reduction gear train, which includes at least one worm fixed to the said driving shaft and meshing with a first set of teeth of an intermediate gear wheel or pinion. A second set of teeth of the intermediate gear wheel meshes with a driven gear wheel or pinion which is fixed to and carried by the driven shaft. In that arrangement, the worm cooperates with a first set of teeth of the intermediate pinion, these teeth being helical. This results in the application of a force having a large axial component which urges the intermediate pinion in a direction such that it tends to be urged away from the plate element on which it is mounted for rotational movement.
As a result of this, in the arrangement disclosed in the above mentioned French patent specification, the reduction gear unit has a cover plate which serves as an abutment means to take the thrust resulting from the above mentioned axial component of force, thereby preventing any excessive displacement of the intermediate gear wheel in the direction of that force. This does however call for a cover plate which is robust and rigid enough for that purpose, so that it must be made of metal having a large enough thickness. This in turn increases the selling cost of the motorised reduction gear unit.
In addition, a finger is provided in cooperation with the intermediate gear wheel in order to make the abutting contact against the cover plate. This finger is an additional component which adds complication to the operation of assembling the motorised reduction gear unit, and again increases its selling cost.